NAVFAC MIL-HDBK-1004/6 Lightning Protection

In accordance with para. 3, Appendix A, determine actual D1 distance from thecenter-line of the building or complex to row of masts on the diagonal axis.With C = 25 ft, D is equal to 82 ft.After the height and location of the masts have been determined,proceed as follows:a) Draw a plan of the building to scale. Locate the masts atdistances which are determined by the method shown in Appendix A, A-3,Example 1, Figure 1.b) Draw an elevation of the building to scale. Locate the masts atdistances which are determined by the method shown in Appendix A, A-3. Draw a100 ft striking distance radius from the apex of the masts to the center-lineof the building or complex as shown. Draw two arcs (the normal axis and thediagonal axis clearance arcs) as shown in Appendix A, A-4, from the point ofintersection with the center-line of the building. These arcs must clear thesalient plane.c) Draw a lengthwise scale elevation of the building. Locate themasts as shown in Appendix A, A-5 (refer to para. 4b, Appendix A). No part ofthe protected building should lie outside the zone-of-protection. Appendix A,A-6 through A-11 provide examples which illustrate application of thisprinciple.5. Facilities Other Than Structures. The following criteria apply forfacilities other than structures.a) This type of facility can be protected by using overhead groundcable spanned between two metal towers or poles which are effectively groundedand meet criteria in para. 3, Appendix A.b) In this system, the overhead cable will intercept lightningstrikes and the resulted electrical current will be safely carried to groundvia the cable and supporting towers or poles.c) The design method establishes the height of the horizontalground cable at mid-span to provide the required zone-of-protection for aspecific facility.d) The mid-span sag of the overhead cable must be included when theheight of the supporting towers or poles is determined (see Appendix A, A-2).e) Selection of cables is based on mechanical strength rather thanelectrical considerations. The overhead ground cable should be stranded,non-corrosive, copper coated steel wire. Minimum size shall be 3/8 in.(9.5 mm) high strength (7 No. 8 strands).f) The construction of the towers and their structural members orthe class of poles should be based on considerations of mechanical and windloading stresses.